It is not known for sure whether Elemmírë witnessed the Darkening with his own eyes, but as a Vanya he was most likely present on the slopes of Taniquetil when the light of Valinor was extinguished.

Notes

1

Given that many of the Eldar left Aman almost immediately after the destruction of the Trees, it is not entirely clear how they could all know of Elemmírë's work: the Exiles must surely have left before it was complete. Perhaps Tolkien simply means that all the Eldarwho remained in Aman knew of the lament, or perhaps it later found its way across the Great Sea by some other means - for example, with the armies of the Valar at the end of the First Age.